1. Field of the Invention
This invention relates to a method for making an ordered magnetic alloy, more particularly to a method for making an ordered magnetic alloy including applying a transient heat to a thermally conductive base to cause generation of an in-plane tensile stress in a disordered magnetic alloy layer to thereby order the disordered magnetic alloy layer.
2. Description of the Related Art
A magnetic material, such as FePt alloy or CoPt alloy, can be used for making a high storage density perpendicular magnetic recording medium. In order to achieve high thermal stability for the high storage density perpendicular magnetic recording medium, the magnetic material is required to have a high magnetocrystalline anisotropy energy (Ku), a high out-of-plane coercive field (Hc⊥) and a low in-plane coercive field (Hc//). It is known in the art that the magnetic material having an ordered phase (or L10 phase), i.e., a face-centered tetragonal (FCT) crystal structure with a [001] preferred orientation, exhibits a high out-of-plane coercive field and a low in-plane coercive field.
Conventionally, the ordered phase of the magnetic material can be obtained by subjecting a non-ordered magnetic material to an annealing treatment under a phase-changing temperature higher than 500° C. Since the phase-changing temperature is relatively high, it is problematic to integrate the annealing treatment into an integrated circuit (IC) manufacturing process.
A method of ordering FePt alloy is described by the inventors in Journal of Applied Physics 105, 07A713 (2009), “Ultrahigh-density [001]-oriented FePt nanoparticles by atomic-scale-multilayer deposition” (hereinafter referred to as Document 1). The method disclosed in Document 1 involves the step of depositing a multilayer film of [Fe/Pt/SiO2]n on a SiO2 layer on a silicon wafer using planetary sputtering techniques, followed by subjecting the multilayer film to rapid thermal annealing under 700° C. for a time period ranging from 2 seconds to 6 hours.
In Document 1, a sample (A) of the multilayer film (n=18) annealed at 700° C. for 2 seconds has X-ray diffraction peaks of [001] orientation and [002] orientation of ordered L10-phase FePt. However, sample (A) has a large average grain size and a broad size distribution (about 112±33.6 nm), which is not suitable for making ultrahigh-density perpendicular recording media. In addition, a sample (B) (n=3) annealed at 700° C. for 6 hours has a smaller average grain size and a narrower size distribution (3.9±0.43 nm) as compared to those of sample (A). However, sample (B) has relatively small X-ray diffraction peaks of [001] orientation and [002] orientation of ordered L10-phase FePt and a poor magnetic property. Although the out-of-plane coercive field (Hc⊥) of sample (B) can be improved by increasing the annealing time to 6 hours, the phase-changing temperature is too high to allow the annealing treatment to be integrated into the IC manufacturing process.
T. Narisawa et al. disclose a method of ordering FePt alloy in an article entitled “[001]-oriented nonepitaxial growth in L10-ordered FePt thin film by SiO2 addition and rapid thermal annealing” (Journal of Applied Physics 109, 033918, 2011) (hereinafter referred to as Document 2). The method of Document 2 involves the step of subjecting a multilayer film of (Fe/Pt/SiO2)9˜72 to rapid thermal annealing under a phase-changing temperature ranging from 450 to 800° C. for 5 seconds to 2 hours. The multilayer film is heated to the phase-changing temperature with a heating rate ranging from 10 to 50 K/s. The multilayer film, including a plurality of Fe layers (each having a thickness of 0.16 nm), a plurality of Pt layers (each having a thickness of 0.18 nm) and a plurality of SiO2 layers (each having a thickness of about 0.1 nm), was prepared by sputtering repeatedly and alternately a Fe target, a Pt target, and a SiO2 target on a thermal oxidization silicon substrate.
In Document 2, the multilayer film of an example (1) was heated to 700° C. with a heating rate of 30 K/s and was annealed at 700° C. for 2 hours. The result of example (1) shows that the intensity of the X-ray diffraction peak of [001] orientation increases with an increase in the amount of SiO2 in the multilayer film. The X-ray diffraction peak of [001] orientation reaches a maximum intensity when the amount of SiO2 is 10 vol %. When the amount of SiO2 is between 12 to 15 vol %, the intensity of the X-ray diffraction peak of [001] orientation is considerably decreased. In an example (2), the multilayer film was heated to 500° C. with a heating rate of 46 K/s, and was annealed at 500° C. for one hour. The result of example (2) shows that a volume fraction of L10 domains (Vfct) (see FIG. 1) is rapidly increased from 0 to 0.23 in 1 to 2 minutes after the phase-changing temperature reaches 500° C., and remains substantially unchanged thereafter till the end of the annealing. In an example (3), the multilayer film was heated to 700° C. with a heating rate of 30 K/s, and was annealed at 700° C. for two hours. The result of example (3) shows that the volume fraction of L10 domains (Vfct) (see FIG. 1) is rapidly increased from 0 to 0.85 in about 10 minutes after the phase-changing temperature reaches 700° C., and is gradually increased to close to 1 after annealing for two hours.
The volume fraction of L10 domains (Vfct=0.23) of the annealed multilayer film of example (2) of Document 2, which is annealed at 500° C., is unsatisfactory for application to the high storage density perpendicular magnetic recording medium. Although a higher volume fraction of L10 domains (Vfct=0.85) of the multilayer film of example (3) can be achieved by annealing the multilayer film at 700° C., the phase-changing temperature is too high to allow the annealing of the multilayer film to be integrated into the integrated circuit manufacturing process. In addition, such a high temperature can result in an increase in the capital cost and/or the equipment cost of manufacturing the ordered FePt alloy.
As such, there is a need to further develop a method of making the multilayer film of an ordered magnetic material that can be integrated into the integrated circuit manufacturing process and that can lower the capital and/or equipment cost.